Relaxation tests on rings and part of rings to assess the behavior of Guadua angustifolia under sustained transverse loading

Relaxation tests of rings, semirings (SR) and C specimens (part of rings like the letter C) of Guadua angustifolia were used to characterize the mechanical behavior of the material under sustained transverse displacement. Tests shorter than one hour showed a behavior typical of viscoelastic materials, described by the initial raise of the force followed by continued relaxation. A generalized linear Maxwell’s model with three branches showed a good fit with experimental data (R2≥0.987). Viscoelastic parameters calculated with two levels of displacement were not statistically different, suggesting that linear viscoelasticity is suitable to characterize the short-time viscoelastic behavior of the material. For displacements maintained over longer times (1–24 h), the force–time relaxation curves showed recoveries and oscillations that viscoelastic theory could not explain. This behavior may be mainly attributed to moisture changes in the samples due to variations of the relative humidity in the lab. Thus, a fitting scheme was developed to consider first the viscoelastic effects and then changes in relative humidity proportional to force variations. The developed algorithm provided a reasonably good fit of the experimental data (mean R2 = 0.870) and the capability to reproduce ups and downs in the force–time curves caused by changes in humidity. The effective circumferential modulus showed substantial reductions over time equal to 49%, 74%, and 41% for the rings, SRs, and Cs, respectively. The study shows the critical role of viscoelasticity and ambient conditions in the performance of bamboo connections.